Importance of diffuse scattering phenomena in moth-eye arrays for broadband infrared applications

Moth-eye (ME) arrays with varying aspect ratios and profile heights were fabricated in Si using a general colloidal lithography and reactive ion etching technique. Antireflective (AR) properties of the arrays were rigorously assessed from the near to far infrared (λ=2-50 μm) using transmission and reflection measurements via dispersive and Fourier transform infrared spectroscopy and modeled using an effective medium approximation (EMA). Infrared transmission of low aspect ratio structures (~2) matched the EMA model, indicating that the most important factor for AR at higher wavelengths is structure height. High aspect ratio structures (>6) were highly transmissive (>90% of theoretical maximum) over a large bandwidth in the mid-infrared (20-50 μm). Specular reflectance, total transmission, and diffuse reflectance (DR) measurements indicate that ME structures do not reach the theoretical maximum at near-infrared wavelengths due to DR and forward scattering phenomena. Ultimately, correlating optical performance with feature geometry (pitch, profile, height, etc.) over multiple length scales allows intelligent design of ME structures for broadband applications.

F. L. Gonzalez, D. E. Morse, and M. J. Gordon
Opt. Lett.
Volume: 39
Number: 1
Pages: 13–16
Date: January, 2014
ICB Affiliated Authors: Michael J Gordon, Daniel E Morse